This invention relates to apparatus for providing position information to a vehicle operating above the surface of the earth in general and more particularly to an improved apparatus of this nature in the form of a correlator or scene matcher which determines position by comparing an internally stored, reference image to a view of the ground, observed radiometrically at microwave frequencies.
Existing position sensing devices operate at either optical or microwave frequencies. While some of the optical units form correlation or matching calculations on an area basis, i.e. without first dissecting the scene and doing an element by element calculation, these sensors cannot operate through clouds or precipitation and are limited to daylight operations. Although microwave sensors have all weather capability, all existing units must first dissect the scene before performing a correlation operation. This leads either to large processing delay when the computation is done serially or to the need for a large computing capacity if parallel data processing is carried out. Furthermore, these devices generally use active microwave devices which illuminate the ground. These types of devices are subject to detection and are hence more likely to be effected by counter-measure tactics.
In view of these deficiencies with prior art correlations, the need for an all weather device which is not subject to detection becomes evident.